Triangular shaped object in STEREO data explained

Some people have noticed a strange triangular or diamond-shape "object" entering the field-of-view of the HI2 telescope on STEREO Behind around
December 26, 2011. You can see the feature in question in this movie moving from right-to-left, just above the trapezoidal occulter on the right side of the image, and more clearly in this close-up movie.
What is this?

The answer lies on the exact opposite side of the image. At the same time as this strange-looking feature starts being visible, the very bright planet Venus enters the HI2-B field-of-view from the lower left. Notice that Venus and the feature stay in step almost exactly opposite each other across the middle of the detector. This is not a coincidence. The strange looking geometrical "object" is actually an internal reflection of the planet Venus within the telescope optics. This effect has been seen many times before. Here's a
particularly striking example of internal reflections caused by the planet Earth as seen early in the STEREO mission, taken from our
image artifacts pages.

Earth is also visible in the December 2011 movie, but is not as bright as Venus. It is the stationary object to the center left. As an added bonus, the tail of Comet Lovejoy can be seen entering from the left towards the end of the movie, just below Venus. Here is a diagram showing the location of the various features in the image: